[llvm] [llvm][NFC] `APFloat`: Add missing semantics to enum (PR #117291)

[Matthias Springer via llvm-commits]

matthias-springer wrote:

Thanks for the fix! Given that `S_PPCDoubleDoubleLegacy` did not previously exist, I think your fix (`llvm_unreachable`) is correct.

https://github.com/llvm/llvm-project/pull/117291